Introduction

Chelating and multidentate amido ligands^1-18 have recently emerged as a viable alternative to the traditionally used cyclopentadienide ligands^19-21 in the chemistry of early transition metals. Complexes of d⁰ metals can be used as catalysts for olefin polymerization,^19-27 silane dehydropolymerization,^28-32 olefin hydrosilylation^33-39 and related transformations. It has been recognized that modifying the electronic and steric environment at the metal center affects significantly the reactivity of the complex, and coordinative unsaturation and high electrophilicity have been invoked as a requirement for increased reactivity in d⁰ systems.

Chapters 2 and 3 described our studies on the structure and reactivity of silylamido yttrium complexes containing the [DADMB]^2- ligand (DADMB = 2,2'-bis(tert-butyl-dimethylsilylamido)-6,6'-dimethylbiphenyl),¹ and their application as olefin hydrosilylation catalysts.³⁹ In a continuing effort to explore the properties of C₂-symmetric chelating silylamides as ancillary ligands for early transition metals, this chapter describes our investigations of a number of zirconium complexes of these ligands, based on biphenyl or binaphthyl backbones.

Results and Discussion

The lithiated silylamine Li₂[DADMB].2THF (1) was prepared as previously described.¹ The analogous binaphthyl compound, Li₂[DMBN].2THF (2), was obtained using the same procedure (eq 1), starting from diaminobinaphthyl (see Experimental Section).

Figure 1. ORTEP diagram of {[DADMB]ZrCl₂}₂ (3).

Reaction of 1 with ZrCl₄ in refluxing THF (eq 2) produced the zirconium complex {[DADMB]ZrCl₂}₂ (3), isolated in 82% yield from pentane solution. The solid state structure of 3 (Figure 1) was determined by X-ray crystallography. Compound 3 crystallizes as a centrosymmetric dimer, with two bridging chloride ligands and one terminal chloride per zirconium. The zirconium centers adopt a distorted trigonal-bipyramidal geometry. The two bridging u-Cl ligands asymmetrically bridge the zirconium centers, resulting in inequivalent Zr-Cl_u bond lengths of 2.575(1) and 2.733(1) Å. For comparison, the terminal Zr-Cl_t bond length is 2.412(1) Å. The close Zr-C_ipso distance (2.529(4) Å) and the small Zr-N-C_ipso angle (93.4(2)^o) suggest the presence of Zr-carbon bonding interactions with one of the biphenyl rings, as is often found in complexes of this type.^3,4 Similar dimeric chloride bridged structures have often been determined or suggested for zirconium dichloride complexes.^9,12,40

The binaphthyl analogue, [DMBN]ZrCl₂.THF (4; DMBN = 2,2'-bis(tert-butyl-dimethylsilylamido)-1,1'-binaphthyl) was prepared in 95% yield from 1 and ZrCl₄ under the same reaction conditions. A notable difference in the properties of 3 and 4 is the tendency of 4 to coordinate one equivalent of THF, which could not be fully removed even after repeated recrystallization.

The reactivity of 3 and 4 as olefin polymerization catalysts when activated with MAO (500 equiv) was investigated. Both complexes were active towards ethylene polymerization (at 40-50 psi C₂H₄, toluene solution, room temperature), with activities of 4.59 kg/(mol.h.atm) for 3 and 4.17 kg/(mol.h.atm) for 4, but no measurable polymer formation was observed with other olefins (1,3-butadiene, propylene) under the same conditions. This rather low reactivity is similar to that of other MAO-activated amido complexes (cf. 64 kg/(mol.h) for {Me₂Si(NCMe₃)₂}ZrCl₂(THF)₂,¹² 13 kg/(mol.h.atm) for {(C₆H₃)₂-2,2'-(NCH₂C₆H₄^tBu-4)₂-6,6'-Me₂}Zr(CH₂Ph)₂,⁸ 2.9 kg/(mol.h.atm) for [Ti(Me₃SiNCH₂CH₂NSiMe₃)Cl₂]⁴¹ ) but is much lower than the best group 4 metallocene / MAO polymerization catalysts, which show activities often exceeding 1 x 10⁵ kg/(mol.h),^20,21 and lower than many catalysts with amido or alkoxide ligands (cf. {Zr[RN(Me₂SiCH₂CH₂SiMe₂)NR](NMe₂)₂} (R = 2,6-Me₂C₆H₃) 990 kg/(mol.h.atm),²⁶ 2,2'-S(4-Me,6-^tBuC₆H₂O)₂TiCl₂ 4740 kg/(mol.h),²⁷ {C₅Me₄SiMe₂N^tBu}ZrCl₂ up to 2750 kg/(mol.h)⁴²)

Attempts to isolate alkyl derivatives of 3 by reaction with MeMgBr, MeLi or PhCH₂MgBr resulted in the formation of yellow oils that were difficult to handle and characterize, due apparently to the very high solubility of the alkylated products. The analogous complexes of 4, however, were found to be less soluble and thus easier to isolate and characterize. The benzyl derivative [DMBN]Zr(CH₂Ph)₂ (5) was obtained as a yellow foamy solid by reaction of 4 with two equivalents of PhCH₂K (eq 3).

The benzylic protons in 5 give rise to two doublets in the ¹H NMR spectrum at 2.24 and 2.09 ppm (²J_HH = 10 Hz), with the ZrCH₂Ph carbon appearing at 72.1 ppm in the ¹³C NMR spectrum (¹J_CH = 110 Hz). These NMR shifts are consistent with h¹-coordination of the benzyl group and within the typical range reported for Zr benzyl complexes.^{11-13,24,26,27,42,43}

The methyl derivative [DMBN]ZrMe₂.THF (6) was isolated by reaction of 4 with two equivalents of MeLi as a black foamy solid (apparently contaminated with dark colored byproducts, not observable by NMR). The methyl groups in 6 give rise to a singlet at 0.55 ppm in the ¹H NMR spectrum, with the carbon atom appearing at 47.0 ppm (¹J_CH = 105 Hz) in the ¹³C NMR spectrum, consistent with the values typically reported for ZrMe species.^11,13,26,27

No reaction was observed between 5 or 6 and ethylene (in benzene-d₆) after 3 h at room temperature, and heating the reaction mixtures at 80 ^oC resulted in decomposition with no evidence for polymer formation. Similarly, no reaction was observed between 5 and PhSiH₃ or H₂ (in benzene-d₆) after 2 days at room temperature, and heating at 80 ^oC resulted in decomposition of 5 in both systems.

While the reaction of 6 with B(C₆F₅)₃ gave an intractable mixture of products, the reaction of 5 with B(C₆F₅)₃ resulted in clean benzyl abstraction (eq 4) and formation of the zirconium complex [DMBN]Zr(CH₂Ph)[h⁶-PhCH₂B(C₆F₅)₃] (7), isolated in 71% yield. The ¹H NMR spectrum of 7 shows the presence of two inequivalent ^tBu groups, which suggests anion coordination leading to a non-C₂ symmetric structure in solution. Such coordination has been often observed in other analogous zwitterionic zirconium species.^{12,13,24,25,27,42-45} The ZrCH₂Ph group gives rise to two doublets at 2.14 and 1.58 ppm (²J_HH = 11 Hz) in the ¹H NMR spectrum, and a signal at 73.6 ppm (¹J_CH = 121 Hz) in the ¹³C NMR spectrum of 7 (cf. 1.95 ppm, 52.5 ppm, ¹J_CH = 122.5 Hz for the ZrCH₂Ph group in {Me₂Si(NCMe₃)₂}Zr(CH₂Ph)[h⁶-PhCH₂B(C₆F₅)₃]¹²). The BCH₂Ph methylene group appears at 2.8-3.2 ppm (br m) in the ¹H NMR spectrum, and the ¹³C NMR shift for the BCH₂Ph carbon is at 38.5 ppm (cf 3.36 and 36.2 ppm in {Me₂Si(NCMe₃)₂}Zr(CH₂Ph)[h⁶-PhCH₂B(C₆F₅)₃]). A variable temperature NMR study of 7 in toluene-d₈ showed that coalescence of the signals from the ^tBu groups occurs at 300 K, which corresponds to an activation energy barrier of 63 kJ mol^-1 (15 kcal mol^-1) for anion dissociation. This value is very similar to that of 13.8 kcal/mol reported for [(1,2-Me₂Cp)₂ZrR][CH₃B(C₆F₅)₃] (R = CH₂CMe₃, CH₂SiMe₃).⁴⁶

The solid state structure of 7 was determined by X-ray diffraction (Figure 2). As usually found in such cationic complexes, the borane anion is closely associated with the metal cation, the PhCH₂B group being coordinated to the Zr atom in an h⁶ fashion. The distance between the plane of the C₆ ring and the Zr atom is 2.325(6) Å, similar to that in other related complexes.^43,47 All Zr-C distances are similar (ranging from 2.675(7) to 2.773(6) Å), which suggests an h⁶ rather than an h⁴ or lower coordination mode. The remaining benzyl group is h¹ coordinated to the metal (Zr-C-C angle of 132.7(4)^o), with no evidence for agostic or Zr - aromatic ring interactions. There are close contacts, however, between the Zr atom and the ipso carbons of both binaphthyl rings, with an average Zr-C distance of 2.567(6) Å and a Zr-N-Cipso angle of 91.5(3)^o.

Figure 2. ORTEP diagram of [DMBN]Zr(CH₂Ph)[h⁶-PhCH₂B(C₆F₅)₃] (7). The C₆F₅ groups of the borane anion have been omitted for clarity.

Reaction of 7 with ethylene (room temperature, 5-10 psi) resulted in fast formation of a new complex within 15 min, presumably an ethylene insertion product, as observed by ¹H NMR (d 0.84, 0.62 (s, 9 H each, ^tBu), -0.24, -0.26, -0.54, -0.76 (s, 3 H each, Me₂Si), other peaks obscured). The formation of ethylene oligomers was also detected after 3 h (by ¹H NMR spectroscopy). Unlike compounds 3 and 4, however, complex 7 produced no measurable amount of polyethylene when tested under the same conditions (without MAO cocatalyst). Similar rapid single insertion of alpha-olefins, but limited polymerization activity has been reported for the analogous complex {Me₂Si(NCMe₃)₂}Zr-(CH₂Ph)[h⁶-PhCH₂B(C₆F₅)₃].¹² This may be contrasted to the high polymerization activity reported for (Me₂C₅H₃)₂ZrMe[MeB(C₆F₅)₃] 6800 kg/(mol.h.atm),^25,45 {Me₃SiN(CH₂CH₂NSiMe₃)₂}Zr(CH₂Ph)[h⁶-PhCH₂B(C₆F₅)₃] 330 kg/(mol.h),¹³ Cp*Zr(CH₂Ph)₂[h⁶-PhCH₂B(C₆F₅)₃] 88 kg/(mol.h),⁴³ and others.^5,47,48

Compound 7 was also observed to react rapidly with 1-hexene (by ¹H NMR spectroscopy), presumably forming an insertion product (d 0.37, 0.78 (s, ^tBu, 9 H each), -0.22, -0.26, -0.40, -0.70 (s, Me₂Si, 3 H each), other peaks obscured), with only a small amount of hexene oligomers found after 1 day. Attempt to conduct the reaction in neat 1-hexene, at room temperature, did not result in formation of measurable amounts of polymer. No reaction occurred between 7 and PhMeC=CH₂.

An attempt to abstract a methyl group from 6 using Ph₃CB(C₆F₅)₄ in dichloromethane-d₂ solution as observed by ¹H NMR resulted in a new set of peaks, assignable to a cationic zirconium species (d 0.99, 0.96 (s, 9 H each, ^tBu), 0.25, 0.22, 0.14 (m, 12 H total, Me₂Si), -0.37 (s, 3 H, ZrMe), other peaks obscured). This in situ generated cation, however, also did not show observable olefin polymerization activity, as no polymer was formed when excess of 1-hexene was added to the reaction mixture (24 h, room temperature). In addition, no polymer formation occurred on exposure of neat 1-hexene to a mixture of 6 and Ph₃CB(C₆F₅)₄.

The reaction of 5 with Ph₃CB(C₆F₅)₄ in benzene-d₆ or dichloromethane-d₂ resulted in a mixture of products, as observed by ¹H NMR spectroscopy. Addition of an excess 1-hexene to this solution did not result in polymer formation (2 h, room temperature). However, a catalyst generated in situ from 5 and Ph₃CB(C₆F₅)₄ (1:1 molar ratio) was found to polymerize hexene when exposed to neat olefin (in the presence of ca. 10% toluene, to improve catalyst solubility). The polymer formed was shown by gel permeation chromatography (GPC) to have an Mw value of 2384 (Mn = 1720) and a polydispersity of 1.39. This molecular weight is much lower than those obtained with other recently reported chelating diamide and alkoxide based catalysts (cf. (MeN(CH₂)₃-NMe)Ti(2,6-ⁱPr₂C₆H₃)₂ + Ph₃CB(C₆F₅)₄ and related systems up to Mw = 239 100,^10,11,49 [(^tBu-d₆)N-o-C₆H₄)₂O]ZrMe(PhNMe₂)[B(C₆F₅)₄] Mn = 45 000,⁵ {1,1'-(2,2',3,3'-OC₁₀H₅SiMePh₂)}ZrCl₂ / MAO Mw = 674 000²⁷ ). The activity of the 5 / Ph₃CB(C₆F₅)₄ catalyst mixture for ethylene polymerization, 5.10 kg/(mol.h.atm), was not significantly different from those for the MAO-activated dichlorides 3 and 4.

Conclusions

A series of zirconium chloride and alkyl complexes with chelating silylamido ligands have been synthesized and studied. The polymerization activity of the MAO-activated dichlorides towards ethylene was found to be relatively low, as compared to both Cp and non-Cp based catalytic systems. Reaction of the benzyl and methyl derivatives with Lewis acids results in alkyl abstraction and formation of zwitterionic species. Anion coordination has been shown to occur both in solid state and in solution for the zwitterionic complex 7. The activity of non-Cp olefin polymerization catalysts has already been seen to depend on multiple factors, including ligand steric bulk and electrophilicity, type of cocatalyst, coordination affinity of the counter-anion in case of cationic species, solvent polarity, etc. It has been observed that MAO-activated catalysts are usually more active than the borane-activated species, in which anion coordination can significantly inhibit polymerization activity, a non-coordinating anion leading to more reactive catalytic site. While the silylamido complexes presented in this chapter exhibit reactivity in agreement with these general trends, they are apparently not promising in terms of application as olefin polymerization catalysts.

Experimental Section

General. All reactions with air-sensitive compounds were performed under dry nitrogen, using standard Schlenk and glove box techniques. Reagents were obtained from commercial suppliers and used without further purification, unless otherwise noted. Olefin-free pentane, benzene, and toluene were prepared by pretreating with concentrated H₂SO₄, 0.5 N KMnO₄ in 3 M H₂SO₄, NaHCO₃ and finally anhydrous MgSO₄. Solvents (pentane, diethyl ether, benzene, toluene, tetrahydrofuran) were distilled under nitrogen from sodium benzophenone ketyl. Benzene-d₆ was distilled from Na/K alloy. ⁿBuLi was used as a 1.6 M solution in hexanes, as supplied by Aldrich, and MeLi as a 1.6 M solution in Et₂O, as supplied by Alpha Aesar. Li₂[DADMB].2THF (1),¹ diaminobinaphthyl,^50,51 B(C₆F₅)₃,⁵² and Ph₃CB(C₆F₅)₄⁵³ were prepared according to published literature procedures. NMR spectra were recorded at 300 or 500 MHz (¹H) with Bruker AMX-300 and DRX-500 spectrometers, or at 100 MHz (¹³C{¹H}) with an AMX-400 spectrometer, at ambient temperature and in benzene-d₆, unless otherwise noted. Signal multiplicities are reported as follows: s - singlet, d - doublet, t - triplet, q - quartet, qn - quintet, m - multiplet. Elemental analyses were performed by the Microanalytical Laboratory at UC Berkeley or by Desert Analytics. Infrared spectra were recorded with a Mattson Infinity 60 MI FTIR spectrometer, as KBr pellets. The molecular weight distributions (vs. polystyrene standards) for polyhexene were measured with a Waters Associates chromatograph equipped with a refractive index detector and a PLgel 5u mixed-D column using THF as a mobile phase.

Li₂[DMBN].2THF (2). DABN (6.90 g, 24.3 mmol) was dissolved in 250 mL of THF and the solution cooled in ice/water bath. Two equivs of ⁿBuLi (32 mL, 51 mmol) was added dropwise with a syringe, with vigorous stirring. The solution turned cloudy and gradually changed from almost colorless through reddish yellow, to bright yellow-orange, with formation of an orange precipitate. After the mixture was stirred at room temperature overnight, a solution of ^tBuMe₂SiCl (8.31 g, 53.5 mmol) in 70 mL of pentane was added. The solution was heated at reflux for 6 h, which resulted in formation of a white precipitate, and was then left to cool slowly overnight. Removal of the volatiles in vacuo yielded a brownish foamy oil, which was extracted twice with pentane (200 and 50 mL). After filtration, the pentane extracts were concentrated to about 50 mL, and 40 mL of THF was added. ⁿBuLi (32 mL, 51 mmol) was added at room temperature, resulting in the formation of a bright yellow-greenish crystalline precipitate. The volatiles were removed and the solid product was washed with pentane (2 x 50 mL) and dried in vacuo to give 15.0 g (92% yield) of 2. ¹H NMR: d 7.53 (m, 6 H), 6.91 (m, 4 H), 6.79 (m, 2 H, binaphthyl H), 2.86 (m, 8 H, THF), 1.11 (s, 18 H, ^tBuMe₂Si), 1.09 (m, 8 H, THF), 0.54 (s, 6 H, ^tBuMe₂Si), 0.13 (s, 6 H, ^tBuMe₂Si). ¹³C{¹H} NMR: d 155.8, 138.7, 128.6, 128.5, 128.1, 127.7, 127.4, 126.5, 122.0, 120.9 (binaphthyl C), 68.5 (THF), 28.7 ((CH₃)₃C), 25.1 (THF), 21.3 ((CH₃)₃C), 1.1 ((CH₃)₂Si), -0.4 ((CH₃)₂Si). IR (cm^-1): 3056 (w), 2952 (s), 2927 (s), 2857 (s), 1618 (s), 1596 (s), 1510 (m), 1469 (s), 1403 (s), 1343 (s), 1285 (s), 1250 (s), 1148 (w), 990 (m), 941 (w), 830 (s), 775 (m), 746 (m). Anal. Calcd. for C₄₀H₅₈N₂Li₂O₂Si₂: C, 71.82; H, 8.74; N, 4.19. Found: C, 71.52; H, 8.76; N, 4.03.

{[DADMB]ZrCl₂}₂ (3). To a mixture of Li₂[DADMB].2THF (1) (3.42 g, 5.72 mmol) and ZrCl₄ (1.40 g, 6.01 mmol) was added 200 mL of THF. The clear, yellow solution was heated at reflux for 20 h. The solvent was removed in vacuo and the resulting oily solid was extracted with pentane (3 x 80 mL). Concentration of the pentane extracts and cooling to -78 ^oC produced a yellow crystalline precipitate, which was isolated and dried in vacuo to give 3.16 g (82%) of 3, containing about 1 equiv of residual THF (by ¹H NMR integration). Recrystallization of 2.88 g of the product from pentane gave 1.35 g of the THF-free complex. ¹H NMR: d 7.17 (d, 2 H), 7.05 (t, 2 H), 6.80 (d, 2 H, aromatic H), 1.87 (s, 6 H, MeAr), 0.96 (s, 18 H, ^tBuMe₂Si), 0.04 (s, 6 H, ^tBuMe₂Si), 0.03 (s, 6 H, ^tBuMe₂Si). ¹³C{¹H} NMR: d 139.4, 138.2, 137.0, 130.4, 129.6, 128.1 (aromatic C), 27.6 ((CH₃)₃C), 21.1 ((CH₃)₃C), -1.6 ((CH₃)₂Si), -3.5 ((CH₃)₂Si). IR (cm^-1): 3052 (w), 2952 (s), 2927 (s), 2856 (s), 1580 (s), 1465 (s), 1305 (s), 1236 (s), 1036 (m), 960 (m), 830 (s). Anal. Calcd for C₂₆H₄₂N₂Si₂Cl₂Zr: C, 51.97; H, 7.05; N, 4.66. Found: C, 51.78; H, 7.24; N, 4.43.

[DMBN]ZrCl₂(THF) (4). A mixture of 2 (2.51 g, 3.75 mmol) and ZrCl₄ (0.92 g, 3.95 mmol) in 80 mL of THF was heated at reflux for 6 h. The volatiles were removed in vacuo and the yellow solid residue was extracted with Et₂O (2 x 50 mL). The filtrate was concentrated to 15 mL and cooled to -78 ^oC. The resulting crystalline precipitate was washed with pentane and dried in vacuo, to obtain 2.57 g of product in two crops (95% yield). ¹H NMR: d 7.62 (m, 2 H), 7.51 (m, 2 H), 7.43 (m, 2 H), 7.08 (m, 2 H), 7.01 (m, 2 H), 6.91 (m, 2 H, binaphthyl H), 3.61 (m, 4 H, THF), 1.39 (m, 4 H, THF), 0.994 (s, 18 H, ^tBuMe₂Si), 0.15 (s, 6 H, ^tBuMe₂Si), -0.25 (s, 6 H, ^tBuMe₂Si). ¹³C{¹H} NMR: d 138.9, 134.4, 132.8, 131.8, 129.6, 129.0, 128.9, 128.1, 127.6, 126.3 (binaphthyl C), 70.1 (THF), 27.7 ((CH₃)₃C), 25.8 (THF), 19.9 ((CH₃)₃C), -1.6 ((CH₃)₂Si), -2.4 ((CH₃)₂Si). IR (cm^-1): 3056 (w), 2953 (s), 2927 (s), 2882 (m), 2855 (s), 1618 (s), 1596 (s), 1509 (m), 1469 (s), 1404 (s), 1391 (s), 1344 (s), 1285 (s), 1250 (s), 1211 (m), 1148 (m), 993 (s), 938 (s), 831 (s), 812 (s), 776 (s), 746 (s), 672 (m). Anal. Calcd. for C₃₆H₅₀N₂OSi₂ZrCl₂: C, 58.03; H, 6.76; N, 3.76. Found: C, 56.45; H, 6.69; N, 3.50. Satisfactory elemental analysis data could not be obtained due to partial desolvation.

[DBMN]Zr(CH₂Ph)₂ (5). A mixture of 4 (0.60 g, 0.80 mmol) and KCH₂Ph (0.22 g, 1.68 mmol) was dissolved in 25 mL of benzene at room temperature. The red insoluble KCH₂Ph was consumed within 20 min resulting in the formation of a cloudy yellow solution. After 45 min the benzene was removed in vacuo and the solid residue was extracted with hexanes (2 x 30 mL). The filtrate was concentrated to 10 mL and cooled to -78 ^oC to give a voluminous oily yellow precipitate. The product was isolated by filtration at -78 ^oC and dried in vacuo to give 0.31 g of a yellow foamy solid (50% yield). The lack of crystallinity prevented purification of the product by recrystallization. ¹H NMR: d 7.61 (m, 4 H), 7.32 (m, 2 H), 7.22 (m, 3 H), 7.06 (m, 4 H), 6.99 (m, 2 H), 6.89 (m, 2 H, aromatic H), 2.24 (d, 2 H, ²J_HH = 10 Hz, ZrCH₂Ph), 2.09 (d, 2 H, ²J_HH = 10 Hz, ZrCH₂Ph), 0.68 (s, 18 H, ^tBuMe₂Si), -0.06 (s, 6 H, ^tBuMe₂Si), -0.19 (s, 6 H, ^tBuMe₂Si). ¹³C{¹H} NMR: d 144.8, 140.1, 134.7, 131.5, 131.1, 130.2, 130.0, 129.2, 128.7, 128.7, 127.9, 127.0, 125.6, 123.4 (aromatic C), 72.1 (ZrCH₂Ph, ¹J_CH = 110 Hz), 27.3 ((CH₃)₃C), 19.9 ((CH₃)₃C), -1.2 ((CH₃)₂Si), -2.3 ((CH₃)₂Si). IR (cm^-1): 3055 (w), 3016 (w), 2951 (s), 2927 (s), 2881 (m), 2854 (s), 1617 (w), 1593 (s), 1470 (m), 1389 (w), 1343 (m), 1249 (s), 1207 (s), 1146 (m), 1030 (m), 991 (s), 938 (m), 865 (m), 834 (s), 810 (s), 744 (s), 697 (m), 670 (m). Anal. Calcd. for C₄₆H₅₆N₂Si₂Zr: C, 70.44; H, 7.20; N, 3.57. Found: C, 67.70; H, 7.16; N 3.78.

[DMBN]ZrMe₂(THF) (6). To a solution of 4 (0.60 g, 0.80 mmol) in 30 mL of Et₂O was added 1.1 mL of MeLi (1.7 mmol) at room temperature. The mixture turned dark brown within 30 min. After stirring for 1 h, the volatiles were removed in vacuo and the resulting black solid was extracted with a 1:1 hexanes / benzene mixture (75 mL). The filtrate was dried in vacuo to obtain 0.41 g of dark brown foamy solid. The lack of crystallinity prevented purification of the product by recrystallization. ¹H NMR: d 7.6 (m, 6 H), 7.0 (m, 4 H, binaphthyl H), 3.56 (m, 4 H, THF), 1.24 (m, 4 H, THF), 0.98 (s, 18 H, ^tBuMe₂Si), 0.55 (s, 6 H, ZrMe₂), 0.22 (s, 6 H, ^tBuMe₂Si), -0.30 (s, 6 H, ^tBuMe₂Si). ¹³C{¹H} NMR: d 138.6, 134.6, 131.9, 131.1, 129.7, 128.9, 128.7, 128.5, 128.3, 127.9, 127.6, 127.4, 125.6, 124.5 (binaphthyl C), 69.3 (THF), 47.0 (Zr(CH₃)₂, ¹J_CH = 105 Hz), 27.6 ((CH₃)₃C), 25.6 (THF), 19.6 ((CH₃)₃C), -1.4 ((CH₃)₂Si), -2.8 ((CH₃)₂Si). IR (cm^-1): 3056 (w), 2951 (s), 2928 (s), 2880 (s), 2853 (s), 1616 (w), 1590 (w), 1500 (w), 1470 (m), 1422 (m), 1345 (m), 1261 (s), 1249 (s), 1224 (s), 1148 (m), 1034 (m), 998 (s), 939 (m), 834 (s), 775 (s), 748 (s), 677 (m). Anal. Calcd. for C₃₈H₅₆N₂Si₂ZrO: C, 64.80; H, 8.01; N, 3.97. Found: C, 56.95; H, 6.72; N, 3.72.

[DMBN]Zr(CH₂Ph)[h⁶-PhCH₂B(C₆F₅)₃].3.5C₆H₆ (7). A mixture of 5 (0.169 g, 0.215 mmol) and B(C₆F₅)₃ (0.11 g, 0.215 mmol) was dissolved in 15 mL of benzene. The solution immediately turned bright orange. After the mixture was stirred for 30 min at room temperature, the solvent was removed in vacuo and the remaining orange powder was washed with hexanes and dried to obtain 0.24 g of product (71 % yield). ¹H NMR: d 8.05 (m, 1 H), 7.70 (m, 2 H), 7.19 - 7.41 (m, 5 H), 7.02 (m, 4 H), 6.90 (m, 3 H), 6.80 (m, 2 H, aromatic H), 2.8 - 3.2 (br m, 2 H, BCH₂Ph), 2.14 (d, 1 H, ²J_HH = 11 Hz, ZrCH₂Ph), 1.58 (d, 1 H, ²J_HH = 11 Hz, ZrCH₂Ph) 0.83 (br s, 9 H, ^tBuMe₂Si), 0.76 (br s, 9 H, ^tBuMe₂Si), -0.27 (s, 6 H, ^tBuMe₂Si), -0.54 (s, 6 H, ^tBuMe₂Si). ¹³C{¹H} NMR: d 150.1, 150.0, 150.0, 148.2, 148.1, 140.4, 138.8, 136.9, 129.3, 128.9, 128.7, 128.6, 128.3, 128.2, 128.1, 128.0, 128.0, 128.0, 127.9, 127.2, 124.2 (aromatic C), 73.6 (ZrCH₂Ph, ¹J_CH = 121 Hz), 28.0 ((CH₃)₃C), 27.5 ((CH₃)₃C), 20.0 ((CH₃)₃C), 38.5 (BCH₂Ph), 0.24 ((CH₃)₂Si), -0.72.((CH₃)₂Si). IR (cm^-1): 3050 (w), 2956 (s), 2932 (s), 2898 (m), 2859 (s), 1640 (w), 1598 (s), 1458 (s), 1382 (w), 1263 (s), 1208 (m), 1151 (m), 1084 (s), 981 (s), 935 (m), 835 (s), 814 (s), 776 (m), 750 (m), 677 (m). Anal. Calcd. for C₈₅H₇₇N₂Si₂ZrBF₁₅: C, 65.04; H, 4.94; N, 1.78. Found: C, 58.93; H, 4.46; N 2.16. Loss of solvent of crystallization is likely responsible for deviation in the elemental analysis data.

Ethylene polymerization. A sample of the zirconium complex (ca. 0.03 mmol) and a 500-fold excess of MAO (ca. 15 mmol) were dissolved in 20 mL of toluene and the solution was transferred to a high-pressure glass reaction vessel. The mixture was pressurized with ethylene at 40-50 psi for 1 h at room temperature. The reaction was stopped by venting the ethylene gas and pouring the solution into a mixture of 100 mL CH₃OH, 100 mL H₂O and 50 mL conc. HCl. The precipitated polyethylene was separated by filtration, washed several times with CH₃OH, H₂O and acetone, and dried under vacuum overnight.

1-hexene polymerization. A sample of 5 (7.0 mg, 0.01 mmol) and Ph₃CB(C₆F₅)₄ (10 mg, 0.01 mmol) was dissolved in 0.2 mL of toluene. To the resulting bright orange solution was added 1.0 g of neat 1-hexene. The mixture spontaneously heated up. The reaction was stopped after 90 min by addition of 5 mL of THF containing a few drops of conc. HCl. The polymer formed was washed twice with 30 mL of H₂O and dried in vacuo for 24 h at room temperature to give 0.78 g of polyhexene (78% isolated yield) as viscous colorless oil. ¹H NMR (chloroform-d): d 1.26 (m), 1.07 (m), 1.01 (m), 0.90 (m), olefinic signals at 5.36, 4.70. ¹³C{¹H} NMR (chloroform-d): d 40.46, 34.81, 32.60, 28.93, 23.46, 14.41, olefinic signals at 125.5, 128.4, 129.2.

X-ray structure determinations. X-ray diffraction measurements were made on a Siemens SMART diffractometer with a CCD area detector, using graphite monochromated Mo-Kalpha radiation. The crystal was mounted on a glass fiber using Paratone N hydrocarbon oil. A hemisphere of data was collected using omega scans of 0.3^o. Cell constants and an orientation matrix for data collection were obtained from a least-squares refinement using the measured positions of reflections in the range 4 < 2theta < 45^o. The frame data were integrated using the program SAINT (SAX Area-Detector Integration Program; V4.024; Siemens Industrial Automation, Inc.: Madison, WI, 1995). An empirical absorption correction based on measurements of multiply redundant data was performed using the programs XPREP (Part of the SHELXTL Crystal Structure Determination Package; Siemens Industrial Automation, Inc.: Madison, WI, 1995) or SADABS. Equivalent reflections were merged. The data were corrected for Lorentz and polarization effects. A secondary extinction correction was applied if appropriate. The structures were solved using the teXsan crystallographic software package of the Molecular Structure Corporation, using direct methods, and expanded with Fourier techniques. All non-hydrogen atoms were refined anisotropically and the hydrogen atoms were included in calculated positions but not refined unless otherwise noted. The function minimized in the full-matrix least-squares refinement was Sigma w(|Fo|-|Fc|)². The weighting scheme was based on counting statistics and included a p-factor to downweight the intense reflections.

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